The present invention relates generally to magnetometers, and more particularly, to fiber-optic magnetic gradiometers for sensing both ac and dc magnetic-field gradients.
A highly sensitive optical-fiber magnetometer with a minimum detectable magnetic field of .about.3.times.10.sup.-9 Oe for 1 m of sensor fiber was demonstrated in the paper "Characteristics of Fiber-Optic Magnetic Field Sensors Employing Metallic Glasses, by Koo and Sigel, Jr., Optics Letters, Vol. 7, No. 7, July 1982. See also "Optical Fiber Magnetic Field Sensors," by A. Dandridge, A. B. Tveten, G. H. Sigel, Jr., E. J. West, and T. G. Giallorenzi, Electronics Letters, Vol. 16, pp. 408-409, 1980. This new class of fiber interferometric sensors depends on the precise measurement of magnetostrictively induced changes in the optical path length of a metal-jacketed fiber. However, for some applications where small local magnetic perturbations in the presence of a strong spatially uniform background magnetic field are to be measured, a magnetic gradiometer is desired. A magnetic gradiometer is essentially a dual-sensor device which enables a differential magnetic-field measurement between the two sensor heads or arms. As a result, temporal magnetic noises which are spatially uniform across the two sensor heads are suppressed. In an ideal magnetic gradiometer where a differential technique is used, one measures only local spatial variations of magnetic fields with infinite common-mode (background field) rejection capability.
The usefulness of a differential or gradient magnetic-field measuring device depends largely on how well the device can be balanced (zero output) in a uniform field environment. In an interferometric fiber magnetic gradiometer, magnetostrictive materials are used to stretch both arms of the fiber interferometer. To achieve exact balance, the amount of stretch in each of the two fiber arms must be equal. This requires a complete match of the magnetic performance characteristics between the two fiber arms, implying both a tight control on the variation of magnetostrictive responses of the magnetic material and the precise match in the lengths of the two sensor elements. It is obvious that in situ fine tuning of these parameters after device assembly is very desirable if not absolutely necessary.